The detection of radiation can be accomplished by a device known as a scintillation detector. This device is generally useful in such diverse fields as medical diagnostic methods and Industrial analysis, basic nuclear research and also nuclear well logging. In all cases, the primary component of a scintillation detector is a material that emits light after interaction with incident high energy radiation, in particular X-rays or Gamma ray photons. The material that exhibits light emission is typically a crystalline solid and preferably a monocrystalline solid that is referred to as a scintillation crystal.
Scintillation detectors have been used in well logging for many years. During the initial use of these scintillators for downhole applications several service companies resorted to the use of dewar flasks to limit the temperature seen by the scintillation detector and in particular by the photomultiplier (PMT) and the associated electronics. Schlumberger tools rated at temperatures up to 200° C. did not use such flasks due to the use of more advanced PMTs and electronics for NaI(TI) scintillation detectors. For scintillation materials such as BGO (Bismuth Germanate, Bi4Ge3O12), it was necessary to resort to dewar flasks and large internal thermal mass to avoid excessive scintillator temperatures that, while not damaging the scintillator, led to unacceptable performance.
The recent advent of new scintillation materials with outstanding temperature performance has made it possible to make detectors capable of working at temperatures even beyond 200° C. if suitable photomultipliers or other devices for conversion of light to electrical signals are employed.
The new scintillators are described in several publications, patents and patent applications. Various patents for LaBr3:Ce and LaCl3:Ce are held by Saint Gobain (e.g. U.S. Pat. Nos. 7,067,815 and 7,233,006), and mixed La-halides are the subject of a patent assigned to General Electric Company (U.S. Pat. No. 7,084,403). There are also several oxide-based scintillators with excellent high temperature performance, which are not hygroscopic. These include LuAP:Ce, LuYAP:Ce, LuAG:Pr and LPS (Lutetium Pyro-Silicate, Lu2Si2O7) to name a few.
The recent observation that some of the new materials, while capable to operate at very high temperature, will tend to crack or shatter as they are heated up or cooled down rapidly, necessitated a new kind of thermal protection, which is focused on an even temperature distribution around the scintillator and on reducing the rate at which the temperature increases without regard to protecting the scintillation detector from excessive temperatures.
The present invention intends to overcome this limitation through the use of new scintillation materials that are less prone to developing thermal stresses and more resistant to them.